Introduction

Within This Page

The Executive House in Chicago is generally known as the first reinforced concrete skyscraper. At the time of it's completion in 1959, it was the tallest reinforced concrete building in the United States at 39 stories, or 371 feet. In 1962, the twin towers of Marina City in Chicago set a new record at 588 ft above grade. These distinctive circular, reinforced concrete towers also served as an early example of a cast-in-place concrete wall system. Subsequently, Chicago's Lake Point Tower built in 1968 and Houston's One Shell Plaza built in 1970 set new records at 645 ft and 714 ft, respectively. While both of the latter buildings are clad in materials other than concrete, their innovative structural systems are reflected in their façades and established the precedent for many of the cast-in-place concrete wall systems seen throughout the United States.

Description

A cast-in-place concrete wall system is an exposed structural system that also serves as the façade. Openings or penetrations in the structural system are generally infilled with windows, masonry, or some other cladding material.

Fundamentals

Cast-in-place concrete wall systems are generally defined by the building's structural system, which consists of the vertical (gravity) load resistant system and the lateral (wind and seismic) resistant system. The vertical load resistant system may be further subdivided into the horizontal framing (floor system) and the vertical framing (column and walls). The lateral resistant system includes moment resisting frames, shear walls, braced frames, or a combination of these systems.

A concrete structure designed and constructed in the United States is governed by the minimum provisions of the ACI Building Code. While most of the design provisions of the Code dictate minimum strength (safety) requirements, the Code also prescribes serviceability and durability requirements. Certain factors which influence the design of the structural system also impact the exterior wall. These factors include deflection, cracking, concrete cover, and corrosion protection.

Performance Issues

Thermal Performance

Cast-in-place concrete walls derive their thermal performance characteristics primarily from the amount of insulation placed in the cavity or within the backup wall.

Moisture Protection

The most common moisture protection system used with cast-in-place concrete wall systems is a barrier system incorporating an adequate joint seal. In some cases where additional moisture protection is needed, the application of a sealer or a concrete coating is also used. Sealers can be either clear or pigmented if used as an enhancement of the precast appearance. Film-forming coatings usually offer a higher level of performance but will have a significant impact on the appearance of the precast concrete unit.

The cast-in-place concrete wall should also be designed to provide the appropriate level of durability for the planned exposure. Durability can be improved by specifying minimum compressive strengths, maximum water to cement ratios, and an appropriate range of entrained air.

Fire Safety1

At relatively high temperatures experienced in fires, hydrated cement in concrete will gradually dehydrate, reverting back to water (steam) and cement. This results in a reduction in strength and modulus of elasticity (stiffness) of concrete that, in some instances, can result in spalling. The overall fire resistance of concrete is influenced by aggregate type, moisture content, density, permeability, and nominal thickness. "Carbonate" aggregates, such as limestone, dolomite, and limerock, have been found by some to improve the overall fire resistance of concrete due to their ability to absorb some of the heat from a fire. Similarly, concretes with lower unit weights (densities) will also offer improved fire resistance, as will dried light-weight concrete. In contrast, concrete with relatively low moisture content, low water-cement ratio, and highly impermeable concrete may spall when subjected to fire.

Acoustics

A cast-in-place concrete wall system and precast concrete panel façade will provide similar performance regarding sound transmission from the exterior to the interior of the building. See Precast Concrete Wall Systems for additional information, as well as the industry and trade association web site links listed at the end of this section.

Material/Finish Durability

The key issue to be addressed in design of a cast-in-place façade element is durability related to environmental exposure such as moisture, carbonation of concrete, and other factors that can contribute to the distress and deterioration of concrete.

Concrete deterioration may occur for two principal reasons: corrosion of the embedded steel resulting in concrete deterioration, and degradation of the concrete itself. Concrete normally offers protection to embedded reinforcing steel through its alkalinity.

Concrete deterioration due to corrosion of embedded steel is usually related to moisture, and is typically in the form of cracking and delamination of the concrete. Where embedded reinforcing steel is not protected by the alkaline environment of the concrete, and the steel is exposed to moisture, corrosion occurs. The corroded steel expands significantly in volume, which results in expansive forces on the adjacent concrete, causing it to crack and spall. This is visually apparent in cracking and delamination of the concrete and rust staining at the location of embedded steel.

Carbonation results in the loss of alkalinity within the concrete to the level of the reinforcing steel. Carbonation normally occurs only in the vicinity of the exposed surface of the concrete, but in some cases may extend to the level of the steel. Once this occurs, the concrete offers no protection to the embedded reinforcing steel, and corrosion begins. Carbonation occurs from a combination of moisture and carbon dioxide.

Corrosion of embedded reinforcing steel is often due to calcium chloride added to the concrete as an accelerator during original construction or later from de-icing salts used in northern climates. Chloride ion in combination with moisture results in corrosion of the embedded steel and resulting deterioration of the surrounding concrete. Sea water, or other marine environments, provide large amounts of chloride.

The exposed surface of the concrete is also vulnerable to weathering from the elements. This may typically be observed as erosion of the concrete paste. Especially in northern regions where precipitation has been found to be highly acidic, exposure has resulted in more significant erosion of the paste on the exposed surfaces.

Freeze-thaw damage results from the freezing of concrete that is saturated with water. This type of damage appears as degradation of the surface, including severe cracking, extending into the concrete. It was accidentally discovered that portland cement concretes that incorporate microscopic air bubbles provided resistance to cyclic freezing and thawing. The air-entrainment provides "relief valves" that protect the concrete. Air-entraining agents are now generally (but not always) added to cement or concrete used in exposed applications that are in areas of the United States subject to sub-freezing temperatures.

Alkali-aggregate reactions result when alkalis normally present in cement react with silicious aggregates in concrete that is exposed to moisture. The reaction produces a toothpaste-like gel that develops over years or decades until the forces created expand and crack the concrete. Most such deleterious aggregates can be detected by experience or test, and low-alkali cements can be used in new construction to prevent significant reactions.

Sulfate attack is produced by the reaction of excessive amounts of sulfate salts with cement components that are exposed to moisture. The reaction leads to the development of expansive forces that eventually crack the concrete. The sulfate salts may come from the environment (e.g., sulfate waters or solids), or from one or more of the concrete constituents (e.g., aggregates, cement, or proprietary product providing fast set).

Other forms of concrete deterioration exist, including freeze-thaw damage, alkali-aggregate reaction and sulfate attack, but less common in cast-in-place concrete wall systems.

Maintainability

Durability of concrete and resistance to deterioration is dependent on durability, proper design, and good workmanship. This will also be true for the materials used to repair existing concrete. A mix design for durable replacement concrete should utilize materials similar to those of the original concrete mix and include air-entrainment, appropriate selection of aggregates, and adequate cement and water content. Good workmanship should address proper mix, placement, and curing procedures. In all cases, a good mix design will enhance good workmanship for durable repair concrete.

In designing repairs to existing concrete, parameters must be established to define the goals of the project based on visual evaluation and laboratory studies. A key concern is the aesthetics of the repair, to match the existing concrete as closely as possible both visually and structurally. Another governing concern is to select repair interventions that retain as much as possible of the original material; however, an adequate amount of distressed concrete must be removed to provide a durable repair.

All repairs of existing concrete require proper preparation of the substrate to receive the repair material. This typically includes sandblasting, airblasting, or other appropriate means to provide a clean surface to which the repair can adequately bond. Bonding agents are commonly used on the substrate surface to enhance the bond of the repair. Existing reinforcing steel that is exposed during the repair may require cleaning, priming, and painting with a rust-inhibitive coating. In most cases, the repair area should be reinforced and mechanically attached to the existing concrete. Reinforcement may be regular steel, epoxy-coated steel, or stainless steel, depending on the conditions.

Proper placement and finishing of the repair is important in achieving a match to the original concrete. Appropriate curing is essential for a durable repair; wet curing is recommended to reduce the time of curing and the potential for surface and shrinkage cracking.

The preparation of trial repairs and mockups to refine the repair design, and also to evaluate repair procedures is a wise procedure. Mockups also permit evaluation of the visual and aesthetic acceptability of the repair design.

Because concrete deterioration is primarily the result of moisture penetration, rehabilitation may also entail application of a decorative surface coating or clear penetrating sealer. These water-resistant coatings and sealers should be breathable and alkali-resistant.

Various repair methods and techniques are available in the market today to reduce the rate of corrosion of embedded reinforcing and associated deterioration of the concrete. One method is cathodic protection, which utilizes an auxiliary anode so that the entire reinforcing bar is a cathode. (Corrosion is an electro-chemical process in which electrons flow between cathodic (positively-charged) and anodic (negatively-charged) areas on a metal surface; the corrosion occurs at the anodes.) Cathodic protection is intended to reduce the rate of corrosion that occurs to embedded steel in concrete, which in turn reduces concrete deterioration.

Cathodic protection is only one of many evolving techniques available to protect concrete. Another technique currently available is realkalization, which involves returning the concrete to its natural alkaline state.

Applications

Cast-in-place concrete wall systems have been in use in the United States for many decades. Much of the early development of this construction type occurred in Chicago, primarily due to the influence of the Portland Cement Association and innovative structural engineers like Fazlur Khan. The permanence of this building type is evident by the number of prominent cast-in-place concrete buildings built in the 1950's and 1960's that still exist and remain functional today.

See Appendices for climate-specific guidance regarding building enclosure design.

Emerging Issues

Ordinances related to the maintenance of façades, including cast-in-place concrete wall systems, have been enacted in New York and Chicago. As the inventory of older buildings increases, the maintenance of these building façades and the life safety issues associated with their deterioration will also increase.

The mechanisms that generally contribute to the deterioration of cast-in-place concrete wall systems are well known. Improvements in design standards and repair technology will result in improved performance.

The necessity to make building envelopes blast-resistant has forced reconsideration of the cast-in-place concrete façade as a design option.

Other

About WBDG

WBDG is a gateway to up-to-date information on integrated 'whole building' design techniques and technologies. The goal of 'Whole Building' Design is to create a successful high-performance building by applying an integrated design and team approach to the project during the planning and programming phases.